In a conventional tensioner, a plunger, protruding from a housing, exerts a force on the back of a pivoted tensioner lever having a shoe in sliding contact with the slack, or return, run of a chain in order to maintain tension in the chain. The plunger of the tensioner engages the lever at a location remote from the pivot axis of the lever.
One form of conventional chain tensioner 500, as shown in FIG. 10, comprises a plunger 520, which protrudes retractably from a plunger-receiving hole 511 in a housing 510. A biasing spring 530, biases the plunger 520 in the protruding direction. Wedge-shaped cams 540, in the form of cam chips, slide on oblique guide surfaces 512 formed in grooves the open front end of the plunger-receiving hole 511. These cams have teeth which engage racks 521 formed on opposite sides of the plunger 520. At least two such cams are provided. A cam-biasing spring 550, loosely fitting on the plunger 520, biases the wedge-shaped cams 540, pushing them axially against the guide surfaces so that they are urged, toward the plunger axis, into the plunger-receiving hole 511. The outer end of the cam-biasing spring 550 bears against an inwardly projecting flange of a spacer 560 secured to the front end of the housing 510.
As a chain elongates during use, the chain tensioner 500 controls backlash by allowing the plunger 520 to proceed sequentially, one tooth at a time, in the projecting direction. In this way, the tensioner maintains appropriate chain tension, and prevents foreign noise upon starting of the engine, and whistling sounds which occur as a result of excessive chain tension.
In a prior chain tensioner 500, the backlash distance was controlled in the design process, by selecting an appropriate angle of inclination for the cam-guiding surfaces 512, which determine the direction of movement of the wedge-shaped cams 540.
However, determining the backlash distance by selection of the angle of inclination of the cam-guiding surfaces gave rise to several problems. That is when the angle of inclination of the cam guide surfaces 512, measured between the guide surfaces and the plunger axis, is decreased excessively to increase the backlash distance, the wedging action of the cams 540 becomes excessive, and the cams and plunger are liable to lock together so that disengagement between the cams and the plunger does not occur. Chipping of teeth also occurs due to wear and insufficient strength of the teeth of both the cams 540 and the racks 521.
On the other hand, when the angle of inclination of the cam guide surfaces 512 is increased excessively in order to reduce the backlash distance, the wedging action of the wedge-shaped cams 540 is impaired. Movement of the cams, on the cam-guiding surfaces 512, into the plunger-receiving hole 511 becomes unreliable, and disengagement is liable to occur due to the unreliability of the movement of the cams.
Therefore, when the backlash distance is determined solely by the angle of inclination of the cam-guiding surfaces 512, the backlash distances are limited to an very narrow range by restrictions such as tooth strength and the like, which need to be considered in designing the plunger racks and cams. Accordingly, tensioners of this kind have not been suitable for all types of engines.
For chain tensioners of the type described, a variety of housings with cam-guiding surfaces inclined at different angles corresponding to the desired backlash distances must be made available. Furthermore, because the guide surface inclinations vary, different wedge-shaped cams must be provided for each of the housings. Parts control was difficult, especially since the wedge-shaped cams are small in size, and cams having different angles of inclination are difficult to distinguish from one another. As a result, in the assembly of these chain tensioners, the wrong wedge-shaped cams were sometimes installed.
Accordingly, objects of the invention are to solve the above-described problems; to provide a tensioner in which the backlash distances required for various engines can be set easily in order to prevent foreign wobbling sounds and other chain noises upon starting of an engine, and to prevent whistling sounds due to excess tensioning of the chain; to maintain appropriate tension over a wide range without limitation; and to achieve a significant improvement in the durability of the backlash mechanism.